Variable-rate selective excitation (VERSE) is a pulse reshaping technique that is applied to the RF and gradient slice-select waveforms used in magnetic resonance imaging (MRI) to reduce the RF power of the slice-select pulses. VERSE allows for a variable trade-off between RF and gradient amplitudes and sample duration for each time sample of the pulse. This results in an RF waveform whose peak region is truncated in amplitude and stretched in time, while the corresponding portions of the gradient waveform experience a dip. This reshaping produces a reduction in peak magnitude and specific absorption rate (SAR) of the RF pulses while preserving MR excitation profiles on-resonance.
In some applications, a VERSE algorithm has been applied to multidimensional and parallel-transmit selective excitation pulses, reshaping the RF and gradient waveforms to take advantage of available gradient amplitude and slew rate, to produce the shortest pulse possible without changing the two-dimensional excitation profile. This results in, e.g., two-dimensional selective excitation pulses with optimal bandwidth, and identical two-dimensional excitation profiles to the original pulses on-resonance.
Slice-select pulses can be made quieter by de-rating them—i.e., by reducing gradient slew rate and/or amplitude (along with RF bandwidth), but both of these measures produce longer pulses and thus increase minimum echo time. Other approaches smooth the corners of various gradient waveforms to make them quieter. For instance, gradient crushers around RF refocusing pulses can be smoothed and lengthened while modifying readout bandwidth in fast spin echo sequences. However, these approaches do not change gradients that are played out during an RF pulse, and are not applicable during RF excitation, inversion, or refocusing pulses.
The pulsing of gradients in MRI pulse sequences can be loud, leading to patient discomfort and impaired communication with scan operators. There are pulse sequences that reduce the acoustic noise. Because of difficulty in quieting the slice-select pulse, these sequences are generally volumetric sequences. While slice select gradient pulses can be de-rated to reduce loudness, a longer minimum echo time can be required, and sometimes a narrower pulse bandwidth is also necessary.